In an earlier episode, Chibs says that Jimmy O' gave him those scars. I'm pretty sure Chibs slicing Jimmy's face was meant to be some type of payback. Don't know if it has any Irish gangland meaning though.

The scars are real, its called a glasgow smile, its something that happened to the actor years ago, he was a DJ in scotland, was leaving a pub where he worked and someone tried to get his records and coat, he wouldnt give them over so the guy stabbed him in the face, and he almost died as a result of those cuts.

For the series though they used the scars for part of the back story of chibs and Jimmy O. Thats why Chibs does that to jimmy at the end before sticking them into his neck.